df12.merge(df13)

#  A    B               C            D       sum_C        sum_D
#  ---  -----  ----------  -----------  ----------  -----------
#  foo  one    -0.710095    0.253189    -0.0752683   1.51216
#  bar  one    -0.165891   -0.433233    -0.165891   -0.433233
#  foo  two    -1.51996     1.12321     -2.13829     2.47479
#  bar  three   2.25083     0.512449     2.25083     0.512449
#  foo  two    -0.618324    1.35158     -2.13829     2.47479
#  bar  two     0.0817828   0.00830419   0.0817828   0.00830419
#  foo  one     0.634827    1.25897     -0.0752683   1.51216
#  foo  three   0.879319    1.48051      0.879319    1.48051
# 
# [8 rows by 6 columns]